Orca description

Also known as the killer whale, the orca (Orcinus orca) is the largest member of the dolphin family, and one of the most distinctive of all cetaceans(2)(3)(5)(6)(7). The orca’s strikingly marked black and white body is unmistakable, being black on the upperparts, sometimes with a light grey ‘saddle patch’ behind the dorsal fin, and white on the underparts, lower jaw and undersides of the tail. White lobes extend up the sides of the body behind the dorsal fin, and there is a white, oval patch behind each eye (2)(3)(5)(6)(7). In newborn orcas, the white areas of the body have an orange hue (5).

Both the male and female orca have a broad, rounded head and snout, an enlarged forehead, large, paddle-shaped pectoral fins and a large dorsal fin(2)(3)(5)(6)(7). However, males grow larger than females, and on reaching maturity become stockier and develop disproportionately larger fins, with adult males easily recognised by the tall, erect dorsal fin, which is the largest of any cetacean, growing to an impressive 1.8 metres in height (2)(3)(5)(6). The female orca, in contrast, has a more backward-curving dorsal fin(2)(6), which grows to just 70 to 90 centimetres in height (3)(6). The shape of an orca’s dorsal fin and saddle patch are unique to each individual (6).

A number of different forms of orca have been identified, which specialise in different types of prey, differ in appearance, behaviour and habitat use, do not associate with each other and are not known to interbreed. Studies have also revealed genetic differences between the different forms, and the orca may therefore be split into a number of different subspecies or even distinct species in the near future (1)(8)(9).

Related species

Orca biology

The orca is the world’s largest predator of warm-blooded animals (2), and the only cetacean to routinely hunt marine mammals (5), including seals, sea lions, dolphins and sea otters (3)(5)(7). The orca will even kill large whales (2)(3)(5)(10), and also hunts fish (including sharks and rays), squid, octopuses, sea turtles and seabirds, such as penguins (1)(2)(3)(5)(6)(7). Despite this varied diet, different orca populations often specialise on particular prey. For example, some of the best studied orcas occur in the eastern North Pacific, where three distinct forms have been identified: ‘resident’ orcas feed on fish, particularly salmon; ‘transients’ hunt marine mammals and occasionally seabirds; and ‘offshores’ appear to feed on sharks. These forms also differ in appearance and behaviour, and do not interact (5)(7)(11)(12).

An intelligent and versatile predator, the orca shows a wide range of different hunting techniques. Some orcas work together to ‘herd’ fish, before stunning the prey with strikes from the tail flukes (5)(6)(7)(13), or join together in coordinated attacks on large whales (2)(5). Others chase prey such as tuna or dolphins for long periods, to exhaust them (7), while seals or seabirds resting on ice may be dislodged into the water by rushing to the surface to create a wave (2)(5). Some orcas in Argentina have even learnt to intentionally strand themselves on beaches to reach seals and sea lions on the shore (1)(2)(5)(6)(14), a behaviour which appears to be taught to the young (14). In many areas, orcas have also learnt to follow fishing fleets to feed on discarded fish, or to take prey directly from longlines (7). The orca itself has no natural predators other than humans (5).

The orca is a social species, communicating using a variety of screams, clicks and whistles, as well as through physical behaviours such as breaching, slapping the flippers or tail, and ‘spyhopping’ (bringing the head out of the water). Click-like sounds are also used for echolocation(2)(5). Orca groups usually consist of up to 40 or 50 individuals, although larger numbers may gather when several groups temporarily join together (2)(5).

The group structure of the orca can be quite complex. For example, in ‘resident’ orcas, the basic social unit is a matriline, consisting of a mature female, her adult offspring, and her daughters’ offspring, with individuals in this group having very strong bonds and staying in the group for life, although they mate outside of it (2)(3)(5)(6)(7). Groups of related matrilines which are frequently seen together and share a unique call repertoire are known as a pod, while above this are ‘clans’, which include a number of pods with similar vocal ‘dialects’; these pods are probably distantly related, but may not form a particular social unit. Finally, ‘communities’ are made up of pods which regularly associate but may not necessarily be closely related (2)(5)(7). The social organisation of ‘transient’ orcas is less well known; the basic social unit is still a matriline, but offspring may disperse from the group they were born into, and so groups tend to be smaller and more dynamic than in resident orcas (3)(5)(7). Transient orcas also tend to share a related call repertoire, rather than showing group-specific ‘dialects’, and are also generally silent while foraging (5).

Male orcas reach sexual maturity at around 15 years, but do not become physically mature until about 21, while females reach sexual maturity in their early teens (2)(5)(7). Breeding can occur at any time of year, although in the northern hemisphere births usually peak between October and March (2)(3)(5)(7). The female orca gives birth to a single calf after a gestation period of around 15 to 18 months, and the calf is weaned at 1 to 2 years old (2)(5)(6). Although potentially living for up to 90 years (2)(5)(7), a female orca only produces a calf once every 3 to 8 years, up to the age of about 40 (2)(5)(6)(7). After reaching 40, the female ceases to reproduce, but may instead take on a role as a ‘grandmother’, passing on important experience to younger relatives (6)(15).

Orca range

The orca is found throughout all the world’s oceans, and is believed to be the most widespread mammal species after humans (1)(3)(5). It also occurs in many enclosed or partially-enclosed seas, including the Mediterranean Sea, Gulf of Mexico, Gulf of California, Red Sea and Arabian Gulf (1). Some orca populations may migrate, probably following movements of their preferred prey species (1)(2)(6)(7).

Orca habitat

The orca occurs in virtually every marine region, from polar waters to the equator, and has even been known to enter bays, estuaries and rivers, as well as ice floes. However, it is most commonly recorded in coastal, temperate waters and in areas of high productivity (1)(2)(3)(5)(7).

Orca status

Orca threats

The orca has been hunted in several regions in the past and, although significant commercial hunting has now ceased, small numbers are still killed in whaling operations by some countries. Orcas are also sometimes seen as competitors by fisheries, and individuals are often shot as a result (1)(2)(3)(5)(7)(16). In addition, the orca may occasionally be caught accidentally in trawl and driftnet fisheries (1)(3)(7), while overfishing can reduce its food supply (1)(7). Live captures of orcas for public display in aquaria have occurred in recent decades, although captive breeding success and anti-captivity campaigns mean this has now decreased (1)(2)(3)(5)(16).

Perhaps the greatest threat to the orca comes from the disturbance and degradation of its habitat. As a top predator, the orca is particularly vulnerable to the accumulation of contaminants in its tissues; for example, some populations have been found to carry high levels of chemicals such as PCBs (polychlorinated biphenyls), with potentially negative effects on survival and reproduction (17). Large-scale oil spills can also affect the orca, either directly, or indirectly by reducing the abundance of its prey (1)(5)(7). Disturbance from boats and other man-made underwater noises can affect orca behaviour, disrupt echolocation and social calls, and potentially reduce the ability of orcas to forage effectively (18)(19)(20)(21). This may be of particular concern in areas where whale-watching is becoming increasingly popular (1)(5)(18)(20), and boat traffic can also increase the risk of collisions and injuries (1).

Some regional orca populations are very small and are considered to be under particular threat of extinction (1)(7)(16). For example, the ‘southern resident’ community of Washington and British Columbia is listed as Endangered under the U.S. Endangered Species Act and the Canadian Species at Risk Act (22)(23), while the population in the Strait of Gibraltar has suffered declines in numbers and prey availability, and the small populations in southern African waters are particularly at risk from the impacts of longline fisheries (7).

Orca conservation

Although once feared by some as a dangerous predator, attitudes towards the orca have fortunately changed in recent decades, and this charismatic cetacean has become a much-loved and intensively-studied species (5). It is covered by a range of national and international conservation legislation (4)(24)(25)(26)(27)(28), and the southern resident population of the north-east Pacific Ocean is also covered by a specific recovery plan (29).

The orca would be likely to benefit from appropriate whale-watching guidelines (18)(20), while Marine Protected Areas, for example in which boat traffic is controlled, may also help to reduce disturbance to this species (19). However, perhaps the most urgent need is for further research into the abundance, life history and conservation status of potentially vulnerable regional populations of orcas, many of which have yet to be fully studied (1). It will also be vital to clarify the taxonomy of the orca before the conservation needs of this fascinating marine predator can be more fully understood and any appropriate conservation action taken (1)(8).

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